supermin: If disabled, don't compile in supermin code or check for supermin.

supermin: If disabled, don't compile in supermin code or check for supermin.

If supermin is disabled at compile time and the user just wants to
use the ordinary appliance, there is no need to compile in all
the supermin code, and in particular there is no need to check
for the supermin appliance (which involves running
febootstrap-supermin-helper that probably doesn't exist).

This fixes a warning message observed under Debian w/o supermin:

  sh: febootstrap-supermin-helper: command not found
(cherry picked from commit 4e656a61d40ff51e63aa06d857c40c14ff31ddb9)